As the complexity of photonic integrated circuits (PICs) increases, the physical size and the number of electrical connections, among others, also increase significantly. Unlike electrical integrated circuits, PICs comprising at least one optical waveguide require one or more optical interfaces in order to get photons in or out of the PIC, i.e. to enable optical communication between said at least one optical waveguide and an external optical system such as an optical fiber.
Regarding packaging of an optical subassembly that comprises a PIC and an external optical system, the optical interface between the PIC and the external optical system present several constraints. A first exemplary constraint is the use of a lens or an optical coupling system to maximise the coupling efficiency between the PIC and the external optical system.
A second exemplary constraint is the use of antireflection coatings on all optical surfaces at the optical interface in order to minimise reflections of optical signals back into at least one of the PIC and the external optical system. In this way, coupling efficiency is also enhanced.
A third exemplary constraint is that it is absolutely essential to avoid any contamination in an optical path between the PIC and the optical fiber or external optical system. Not only does this apply to particulate contamination, but most importantly to the avoidance of condensation of water on optical surfaces at the optical interface due to external environmental conditions which can change the optical performance of the optical interface or possibly form a site for corrosion.
In order to provide absolute environmental protection to the optical subassembly and associated optical interface between the PIC and the external optical system taking into account the constraints mentioned above, conventional packaging utilises a hermetic package. In this respect, it is common practice to provide an anti-reflection coated window in the package or a hermetically sealed fiber connected to the package. Implementing more than one hermetically sealed fiber into the package can present both technical and manufacturing problems because normally the fibers need to be mechanically manipulated within the package for the purposes of alignment and fixing.
In either case, i.e. application of a hermetic window or a hermetically sealed fiber, the cost of the hermetic package and associated manufacturing processes and costs will dominate the total cost of the packaged optical subassembly. A requirement to support high numbers of electrical connections as is normal for PICs also adds to the complexity and therefore to the cost of the hermetic package.